Eddy current detection of wheel speed with voltage threshold shifting

ABSTRACT

A wheel speed detection system including a rotator, a sensor head, a detector, a pulse converter, a speed calculator and a threshold shifter. Plural concave and convex portions are formed on a periphery of the rotator rotating together with a wheel. The sensor head includes a coil to generate an alternate current magnetic field. The detector excites the coil to generate an eddy current on the concave and convex portions, and outputs alternate current detection signals corresponding to changes in the eddy current generated with rotation of the rotator. The pulse converter converts the alternate current detection signals into pulse signals according to preset threshold levels. The speed calculator calculates rotational speed of the wheel based on the pulse signals. The threshold shifter sifts the threshold levels corresponding to a difference between a default average and an average of the alternate current detection signals actually outputted from the detector.

TECHNICAL FIELD

This invention relates to a wheel speed detection system for detectingrotational speed of wheels on various types of vehicles.

BACKGROUND ART

Conventionally, as a wheel speed detection system on various types ofvehicles, a wheel speed detection system, for example, wherein amagnetic encoder is used is known, and has been the primary method,especially for automobiles, to detect the speed of a vehicle necessaryfor brake control and others.

Meanwhile, magnetically levitated trains (to be referred to as linearvehicles) with primary side on ground system, wherein a vehicle ispropelled by controlling power supply to propulsion coils on the ground,travel on wheels in a speed level wherein the speed is lower thanpredetermined speed (for example, in the low hundred km/h).Additionally, traveling control on a linear vehicle is basicallyconducted on the ground side for all speed levels. However, if there isa problem in the ground side control, or in a superconductive magnetmounted on the vehicle, for example, speed control from the ground sidemight not be possible. For this kind of possibility, a disc brake devicewhich brakes wheel rotation is mounted on a linear vehicle as anemergency brake.

For controlling this disc brake device, it is necessary to detectrotational speed of wheels, as well as in automobiles mentioned above.However, as well known, wheels on a linear vehicle are positioned in apowerful magnetic field formed mainly by a superconducting magnet, andit is extremely difficult to detect rotational speed accurately using amagnetic sensor such as a magnetic encoder, or a magnetostrictive sensorin such a powerful magnetic field. Besides, for principles of a magneticencoder, a rotator thereof is required to be magnetic. Nevertheless, dueto a limitation that linear vehicles are used in a powerful magneticfield, basically magnetic materials are not used therein, and mounting amagnetic encoder itself is difficult. Thereby, in a linear vehicle,rotational speed of wheels has been conventionally detected by anoptical encoder.

In FIG. 8, the schematic structure of a conventional optical encodertype of wheel speed detection system mounted on a linear vehicle isshown. The wheel speed detection system shown in FIG. 8 is mainlyconstituted with an optical encoder 62 comprising a slit disk 62 a whichrotates together with a support wheel 3 configured with a tire 1 and analuminum wheel (to be referred to simply as wheel) 2, and a signalprocessing device 68 connected to the optical encoder 62 via an opticalcable 67.

The optical encoder 62 is disposed in a wheel axle 60 on one end of anarm 4 constituting a support leg device (to be described in detaillater; refer to FIG. 1). By joining a coupling 65 in the side of theoptical encoder 62 and a coupling 66 in the side of the wheel 2, theslit disk 62 a is rotated along with the rotation of the support wheel(to be referred to simply as wheel) 3.

Then, light from a projector inside the signal processing device 68 isprojected to the slit disk 62 a via the optical cable 67. When the lightis reflected, the reflected light is transmitted via the optical cable67 and received by the signal processing device 68. Based on thepresence/absence of the reflected light and the timing of receptionthereof, the rotational speed of a wheel is detected.

Although the wheel speed detection system by means of the opticalencoder 62 can conduct accurate detection in a vehicle such as a linearvehicle which is in a powerful magnetic field, there are some problems,such as the price of the optical encoder 62 itself being high, or theincapability of accurate detection due to attenuation of light amountcaused by aged deterioration.

Moreover, since the couplings 65 and 66 are respectively coupled inorder to rotate the slit disk 62 a, there is another problem that theworkload in maintenance, such as replacing tires, is increased. That is,to remove the wheel 3 from the wheel axle 60, it is necessary to removethe respective couplings 65 and 66 first. This work consumes a lot oftime in maintaining an entire linear vehicle.

On the other hand, as another wheel speed detection system, other thanmagnetic type or optical type described above, a system by means of aneddy current displacement sensor has been proposed. This system isconstituted so that concavities and convexities are disposed withcertain intervals on the periphery of a rotator, for example, and aneddy current displacement sensor is fixed in a position, a certaindistance away from the concavity and convexity surfaces. The sensordetects that the concave and the convex portions alternately face thesensor corresponding to the rotation of the rotator (wheel rotation),and wheel rotational speed is detected from the detection signals(detection voltage of alternating current) (e.g. refer to Document 1).

Consequently, it can be considered to adopt a wheel speed detectionsystem with this eddy current displacement sensor in a linear vehicle.More specifically, as FIG. 9 shows, concave and convex portion 7 isdisposed on the periphery of inside the wheel 2 (in the vehicle bodyside), and an eddy current displacement sensor (to be referred to simplyas eddy current sensor) 10 is fixed on the arm 4 so as to face theconcave and convex portion 7. Thereby, the concave and convex portion 7which directly face the eddy current sensor 10 change alternately suchas; the concave portion→convex portion→concave portion . . . ,corresponding to the rotation of the wheel 3. Detection voltage inaccordance with this change is transmitted to the signal processingdevice 70 via a cable 15. In the signal processing device 70, thisdetection voltage is converted into pulse signals according topredetermined threshold levels, and rotational speed is calculated fromthe converted pulse signals.

If a system is built as described above to detect wheel rotational speedby using an eddy current sensor 10, it is not necessary to mount asensor in the wheel axle 5, while it is necessary in an optical encodertype. In the support leg device, only fixing the eddy current sensor 10on the arm 4 is required. Hence, when maintenance of the wheel 3 isconducted, there is no need to handle the parts constituting the wheelspeed detection system for joining and detaching the couplings in anoptical encoder type, for example, consequently the workload inmaintenance is decreased. Moreover, since an optical cable is not used,the characteristic problems of optical encoders, such as attenuation ofan optical cable, can be solved.

[Document 1]

Unexamined Japanese Patent Publication No. 2000-121655

However, if a wheel speed detection system using an eddy current sensoris applied to a linear vehicle, from the reasons (1) and (2) describedbelow, there was a problem that the distance between the entire concaveand convex portion 7 and the eddy current sensor 10 (more specifically,the distance d between a surface of a convex portion and the eddycurrent sensor 10; refer to FIGS. 10A and 10B to be described later)which should normally be retained certain distance might be changed.

(1) In a rotator, such as a wheel 3 of a linear vehicle, which supportslarge load, the distance between the eddy current sensor 10 and theentire concave and convex portion 7 changes owing to fluctuation duringrotation or change in the load on the wheel. Particularly in case of alinear vehicle, a wheel center (the wheel axle 5) and a shaft of anactuator in a support leg 6 (not shown in FIG. 9, refer to FIG. 1 to bedescribed later) that receives vertical load are offset. Thus the arm 4is twisted due to the load received from the support leg 6. When theload changes, i.e. the state of the twisting changes because of thechange in the load on the wheel, a change in the above mentioneddistance is caused.

(2) For a linear vehicle, work such as maintenance of the wheel 3 ortire change is conducted relatively frequently. Hence, due to an errorin assembly during maintenance, the distance between the eddy currentsensor 10 and the entire concave and convex portion 7 changes at everymaintenance of the wheel 3 or tire change. Additionally, when a wheel 2itself, wherein the concave and convex portion 7 is disposed, ischanged, owing to production tolerance of the wheel 2, theabove-mentioned distance might be, as expected, changed.

If the distance between the entire concave and convex portion 7 and theeddy current sensor 10 changes as described above, it becomes difficultto accurately detect the wheel rotational speed based on detectionvoltage from the eddy current sensor 10. This mechanism is going to bedescribed based on FIGS. 10A and 10B. FIGS. 10A and 10B are graphsshowing examples of sensor detection voltage and pulse output, in casean eddy current sensor is used as a wheel speed detection system of alinear vehicle.

Firstly, FIG. 10A shows a case when the distance between the eddycurrent sensor 10 and a convex portion 7 a is normal (d=d₀). By rotationof the wheel 3, when the concave and convex portion 7 is moved (rotated)to the direction of an arrow A, the eddy current sensor 10 alternatelyfaces; the convex portion 7 a→the concave portion 7 b→the convex portion7 a . . . . It is to be noted that moving the concave and convex portion7 in the direction of the arrow A with respect to the eddy currentsensor 10, and moving the eddy current sensor 10 in the direction of thearrow A′ with respect to the concave and convex portion 7 aresubstantially the same. Thus, hereinafter, in the descriptions of FIGS.10A and 10B, and in the descriptions of FIGS. 4A, 4B, FIGS. 7A and 7Bwhich are to be described later, it is described that the eddy currentsensor 10 is moved equivalently in the direction of the arrow A′ byrotation of the wheel 3.

Due to the movement of the eddy current sensor 10, detection voltagewith sinusoidal waves as shown in the drawing is obtained. In order toconvert detection voltage into pulse signals, threshold voltages V_(TH)and V_(TL) having hysteresis are set in advance. Thereby, detectionvoltage is converted into pulse signals as shown in the drawing.

However, due to a change in the load on a wheel or an error in assembly,when the distance d between the eddy current sensor 10 and the convexportion 7 a becomes adjacent as shown in FIG. 10B (d=d_(n)<d₀), by thesensing principle of the eddy current sensor 10, detection voltagebecomes small, and the amplitude range of detection voltage might becomesmaller than threshold voltage V_(TH).

Consequently, conversion into pulse signals at the level of thresholdV_(TH) cannot be conducted and, as shown in the figure, pulse signals inthe low level are constantly outputted. Conversely, although it is notshown in the figure, when the eddy current sensor 10 is distant from theconvex portion 7 a (d>d₀), the sensor detection voltage reaches higherlevel than the state in FIG. 10A, and the level of the threshold V_(TL)might become smaller than the amplitude of the detection voltage.Accordingly, pulse signals in the high level are constantly outputted.

The present invention was made in view of the above issues, and itsobject is to be able to detect wheel rotational speed accurately evenwhen the distance between an eddy current sensor and a convex portion ischanged owing to various factors such as a change in load on a wheel oran error in assembly.

DISCLOSURE OF INVENTION

In order to attain the above object, a wheel speed detection system ofthe present invention comprises a rotator which rotates on an axlecenter of a wheel together with the wheel, and plural concave and convexportions formed on a periphery thereof along a rotational direction(circumference direction) with predetermined intervals therebetween; asensor head disposed so as to face a surface of a convex portion withcertain distance therebetween, and constituted with a coil to generatealternate current magnetic field therearound under supply of alternatecurrent; a detector which excites the coil by supplying alternatecurrent to generate eddy current on the concave and convex portions, andoutputs alternate current detection signals corresponding to a change inan amount of eddy current generated with rotation of the rotator; apulse converter which converts the outputted alternate current detectionsignals into pulse signals according to preset threshold levels; and aspeed calculator which calculates wheel rotational speed based onconverted pulse signals.

That is, the wheel speed detection system of the present invention is,on an equality with a wheel speed detection system with a conventionaleddy current displacement sensor, to detect wheel rotational speed basedon a change in the amount of eddy current generated with the rotation ofa rotator. Additionally, the surfaces of a convex portion and the sensorhead are disposed so as to face each other with certain distancetherebetween. In other words, the distance between a surface of aconcave portion and the sensor head is also constant.

However, as described in an issue of the conventional skill, when, forexample, load on a wheel (load on an axle) changes, or when the wheeland the rotator are detached and attached again in a maintenanceoperation of wheels, due to a mechanical twist on members supporting andfixing the rotator and the sensor head, or an error in assembling thesemembers, there is a possibility that the facing distance between thesurface of a convex portion and the sensor head might be changed fromthe above-mentioned certain distance.

Consequently, in the present invention, a threshold shifter shiftsthreshold levels corresponding to the actual facing distance between thesurface of a convex portion and the sensor head. When the thresholdlevels are shifted by the threshold shifter, the pulse converterconducts a conversion into pulse signals according to the thresholdlevels after the shifting. Thereby, even when the distance between thesurface of a convex portion and the sensor head, which is originallysupposed to be retained to be certain distance, is changed, according tothe change, threshold levels can be adjusted to appropriate levels.

Thus, according to the wheel speed detection system of the presentinvention, even when the distance between the surface of a convexportion and the sensor head changes due to a change in load on the axleand the like, the pulse converter conducts conversion into pulse signalsaccording to appropriate threshold levels corresponding to the change.Consequently, wheel rotational speed can be detected accurately.

It is to be noted that a change in the amount of eddy current appears asimpedance change in a coil, in the same manner as in a general eddycurrent displacement sensor, for example. Thereby, alternate currentdetection signals corresponding to the change can be taken out, forexample, as a change in resonant voltage of a resonant circuit includingthe coil.

In addition, formation of concave and convex portions can be done in amanner so that a change in the amount of eddy current corresponding towheel rotational speed can be detected. For example, as described inFIG. 9, the concave and convex portions can be formed on a periphery ofa surface perpendicular to the rotational axis of the rotator. Foranother example, the concave and convex portions can be formed on alateral surface of the rotator recited in aforementioned Document 1(that is, a gear is used as the rotator).

Furthermore, shifting of threshold levels by the threshold shifter canbe constantly conducted. However, in some cases when, for example, thereis little difference between the actual facing distance and theabove-mentioned certain distance, it is acceptable to make a pulse fromalternate current detection voltage without a shifting, according topreset threshold levels. Therefore, the shifting can be conducted onlywhen, for example, it is determined to be necessary to shift thresholdlevels by checking actual alternate current detection voltage.Determination whether or not threshold levels are to be shifted can beconducted arbitrarily depending on the degree in the change of thefacing distance.

With this constitution, a shifting of threshold levels by the thresholdshifter can be done, specifically, so that threshold levels fall withinthe amplitude range of alternate current detection signals outputtedfrom the detector. If threshold levels are at least in the amplituderange of alternate current detection signals, alternate currentdetection signals can be converted into pulse signals according tothreshold levels, and thereby wheel rotational speed can be detectedaccurately.

It is to be noted that, also in this case, determination whether or notthreshold levels should be shifted can be arbitrary conducted. Thresholdlevels can be shifted only when, for example, threshold levels are outof the amplitude range of actual alternate current detection signals(such as a case in FIG. 10B). Additionally, threshold levels can beshifted also when threshold levels are within the amplitude range butnear to the maximum value or the minimum value of alternate currentdetection signals.

Additionally, the degree how much the threshold shifter shifts thresholdlevels can be determined by various methods. It can be determined, forexample, corresponding to the difference between actual alternatecurrent detection signals and alternate current detection signals whenthe facing distance is the above-mentioned certain distance.

In other words, in a wheel speed detection system with thisconstitution, the threshold shifter obtains a difference between adefault average, which is the average of alternate current detectionsignals when the facing distance between the surface of a convex portionand the sensor head is the above-mentioned certain distance, and anaverage of actual alternate current detection signals outputted from thedetector. Corresponding to the difference, the threshold shifter shiftsthreshold levels.

In this case, for example, the shifting amount can be as much as thedifference. For another example, it can be constituted so that in casethe difference is little (that is, in case when the amount of change inthe facing distance is little), the threshold shifter does not shiftthreshold levels, but shifts threshold levels only in case when thedifference is large.

As described above, if the amount of a shifting is determined dependingon the difference between the default average and the average of actualalternate current detection signals, threshold levels can be moreappropriately shifted, and the reliability of the wheel speed detectionsystem can be improved.

As for threshold levels, a method, for example, wherein only one levelis set to determine whether alternate current detection signals arehigher or lower than the level and to make a pulse from alternatecurrent detection signals, is possible. Nevertheless, in considerationof noise tolerance and others, it is more preferable to set twothreshold levels with hysteresis. Consequently in this case, thethreshold shifter preferably shifts both of the two threshold levelswhile retaining the amount of hysteresis.

Next, the wheel speed detection system of the present inventioncomprises a rotator, a sensor head, a detector, a pulse converter, and aspeed calculator. A detection signal shifter shifts alternate currentdetection signals outputted by the detector for some level correspondingto an actual facing distance between the surface of a convex portion andthe sensor head. In this manner, when alternate current detectionsignals are shifted by the detection signal shifter, the pulse converterconverts the alternate current detection signals to the pulse signalafter a shifting.

In other words, in the above-described system, threshold levels areshifted. Contrary, in this case, threshold levels are not shifted, butactual alternate current detection signals themselves are shifted.Substantially equivalent effect to shifting threshold levels in theabove-described system can be obtained.

Therefore, according to the wheel speed detection system of this case,even when a difference is caused between alternate current detectionsignals, when the facing distance is the above-mentioned certaindistance, and actual alternate current detection signals, because of achange in the distance between the surface of a convex portion and thesensor head due to a change in load on axle or the such as, alternatecurrent detection signals can be shifted in the direction to restitutethe difference. Hence, wheel rotational speed can be detectedaccurately.

It is to be noted that, also in this case, shifting alternate currentdetection signals by the detection signals shifter can be constantlyconducted. However, the shifting can be conducted only when, forexample, it is determined to be necessary after checking actualalternate current detection voltage. The determination whether or notshifting should be conducted can be made suitably depending on thedegree of a change in the facing distance.

Specifically, in this configuration, a shifting of alternate currentdetection signals by the detection signal shifter can be conducted, forexample, so that threshold levels can be included within the amplituderange of alternate current detection signals. If alternate currentdetection signals are shifted so as to fall into the amplitude range ofalternate current detection signals, alternate current detection signalsafter a shifting can be converted into pulse signals according tothreshold levels. Hence, wheel rotational speed can be detectedaccurately.

It is to be noted that, also in this case, determination whether or nota shifting of alternate current detection signals should be conductedcan be made accordingly. For example, it can be set to shift alternatecurrent detection signals constantly, or for another example, it canalso be set to shift alternate current detection signals only whenthreshold levels are out of the amplitude range of alternate currentdetection signals (such as a case in FIG. 10B).

Additionally, the degree of shifting alternate current detection signalsby the detection signals shifter can be determined by various methods.Alternate current detection signals can be shifted, for example, forsome level corresponding to the difference between a default average,which is an average of alternate current detection signals when thefacing distance between the surface of a concave portion and the sensorhead is certain distance, and the average of actual alternate currentdetection signals outputted from the detector.

In this case, the shifting amount can be, for example, equivalent to theamount of the difference. Alternatively, for another example, it can beset so that when the difference is little (that is, when the change inthe facing distance is little), a shifting is not conducted, but ashifting is conducted only when the difference is large.

Accordingly, if the shifting amount is determined corresponding to thedifference between a default average and the average of actual alternatecurrent detection signals, threshold levels can be shifted moreappropriately, and the reliability of the wheel speed detection systemcan be improved.

Also in the system of this case, it is possible to set, for example,only one level as a threshold level, and by determining whetheralternate current detection signals are higher or lower than the level,making a pulse from alternate current detection signals can beconducted. However, in consideration of noise tolerance, it is morepreferable to set two threshold levels having hysteresis.

Meanwhile, the wheel speed detection system of the present invention isapplicable to various vehicles. As described above, in a linear vehiclewherein maintenance of wheels which support relatively large load isconducted frequently, it is probable that the above-mentioned facingdistance is changed because of errors in assembly during maintenance, ora change in load on axle.

Consequently, the system of the present invention can be more effectiveto measure rotational speed of wheels disposed on a vehicle, if, forexample, mounted on a railway with primary side on ground system,wherein a vehicle is propelled by magnetic interaction generated betweenpropulsion coils disposed along a track on the ground and a magneticfield system mounted on the vehicle by distributing power to thepropulsion coils. The system of the present invention makes it possibleto build a wheel speed detection system with high reliability in arailway vehicle with primary side on ground system.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing a schematic structure of a supportleg device comprised by a magnetically levitated train;

FIGS. 2A and 2B are block diagrams showing schematic structures of awheel speed detection system of a first embodiment;

FIG. 3 is a flowchart showing a threshold voltage shifting process ofthe first embodiment;

FIGS. 4A and 4B are graphs showing examples of sensor detection voltageand pulse output in the wheel speed detection system of the firstembodiment, and respectively showing a case wherein a sensor and aconvex portion are adjacent (4A), and a case wherein the sensor and aconvex portion are distant (4B);

FIG. 5 is a block diagram showing schematic structure of a wheel speeddetection system of a second embodiment;

FIG. 6 is a flow chart showing a detection data shifting process of thesecond embodiment;

FIGS. 7A and 7B are graphs showing examples of detection voltage andpulse output in the wheel speed detection system of the secondembodiment, respectively showing a case wherein a sensor and a convexportion are adjacent (7A), and a case wherein the sensor and the convexportion are distant (7B);

FIG. 8 is an explanatory view showing a schematic structure of a wheelspeed detection system with an optical encoder mounted on a magneticallylevitated train;

FIG. 9 is an explanatory view showing a schematic structure of a casewherein an eddy current displacement sensor is used as a wheel speeddetection system of a magnetically levitated train; and

FIGS. 10A and 10B are graphs showing examples of detection voltage andpulse output in a case wherein an eddy current displacement sensor isused as a wheel speed detection system of a magnetically levitatedtrain.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings.

First Embodiment

FIG. 1 is a perspective view showing a schematic structure of a supportleg device comprised by a magnetically levitated train (linear vehicle).The linear vehicle, not shown in the drawing, is so called a trainvehicle with primary side on ground system, wherein a superconductingmagnet is mounted in an undercarriage as a magnetic field system, andthe linear vehicle is propelled by controlling power supply topropulsion coils disposed along a track on the ground and by magneticinteraction between propulsion coils and the superconducting magnet inthe vehicle. The linear vehicle floats and travels at and overpredetermined speed (for example, in the low hundred km/h), but travelson wheels in a speed level lower than the predetermined speed. For alinear vehicle to travel on wheels, a support leg device shown in FIG. 1is disposed.

As shown in FIG. 1, in the support leg device of a linear vehicle, awheel (supporting wheel) 3 constituted with a tire 1 and a wheel(aluminum wheel) 2, as a rotator, is attached to an axle (a center ofthe axle) 5 disposed on one end of an arm 4. Moreover, a support leg 6whose one end is connected to the arm 4 is constituted to bestretchable/contractable having an actuator therein. The other ends ofthe support leg 6 and the arm 4 are respectively connected to a bracket,not shown in the drawing. Thus, by driving the actuator in the supportleg 6 and stretching/contracting the support leg 6, the wheel 3 can bemoved up and down.

On the wheel 3, although not shown in the drawing, a disk brake deviceis mounted. The disk brake device is operated to brake a linear vehiclewhen the linear vehicle is traveling on the wheel 3 and in case thevehicle cannot be braked by ground control.

In the present embodiment, a wheel speed detection system comprising aconcave and convex portion 7, an eddy current sensor 10, a cable 15 anda signal processing device 20, is mounted in the support leg device inorder to detect rotational speed of the wheel 3 which is necessary tocontrol the operation of the above-mentioned disk brake device.

The concave and convex portion 7 is constituted, as shown in thedrawing, with convex portions 7 a and concave portions 7 b which arealternately formed having predetermined intervals therebetween on theperiphery of the wheel 2 along the rotational direction thereof. Thematerial of the concave and convex portion 7 in the present embodimentis aluminum. The eddy current sensor 10 is disposed and fixed on the arm4 facing the concave and convex portion 7 with a certain distance (to bereferred to as “default facing distance” hereinafter) d₀. Sensordetection voltage from the eddy current sensor 10 is transmitted to thesignal processing device 20 via the cable 15.

FIG. 2A shows a block diagram illustrating a schematic structure of awheel speed detection system of the present embodiment. As shown in FIG.2A, the eddy current sensor 10 has a detection coil 11 embedded in asensor head 10 a thereof. The distance between the sensor head 10 a anda convex portion 7 a is maintained to be the default facing distance.Moreover, as to be described later, sensor detection voltage (equivalentto the alternate current detection signals of the present invention)corresponding to impedance changes of the detection coil 11 istransmitted to the signal processing device 20 via the cable 15.

FIG. 2B shows a block diagram illustrating a schematic structure of theeddy current sensor 10. The eddy current sensor 10 mainly comprises thedetection coil 11, an oscillation drive device 12 and a wave detectioncircuit 13. The detection coil 11 constitutes a resonant circuittogether with a condenser (not shown) in the oscillation drive device12. By the oscillation drive device 12 supplying alternate current tothe resonant circuit and oscillating the resonant circuit atpredetermined frequency of oscillation, high frequency current flows inthe detection coil 11 and an alternate current magnetic field isgenerated therearound. When the wheel 3 is rotated in the alternatecurrent magnetic field, eddy current is generated in the aluminum wheel2.

In the present embodiment, since the concave and convex portion 7 isformed on the aluminum wheel 2 so as to face the sensor head 10 a, aconcave portion 7 b and a convex portion 7 a alternately face the sensorhead 10 a corresponding to the rotation of the wheel 3, such as aconcave portion 7 b→a convex portion 7 a→a concave portion 7 a . . . .Hence, the amount of eddy current generated in the entire part of theconcave and convex portion 7 changes corresponding to the rotation ofthe wheel 3, and the impedance of the detection coil 11 is changedthereby.

Consequently, the oscillation amplitude of the oscillation circuit, i.e.the amplitude of oscillation voltage outputted from the oscillationdrive device 12 changes corresponding to the rotation of the wheel 3.This oscillation output is picked up in an envelope detection andsuitably amplified in the wave detection circuit 13, subsequentlytransmitted to the signal processing device 20 as sensor detectionvoltage. Specifically, as well as the waveforms shown FIGS. 10A and 10Bdescribed above, when a convex portion 7 a faces the sensor head 10 a,the amount of eddy current becomes large and the impedance change in thedetection coil 11 becomes large, thus the sensor detection voltagebecomes small. On the other hand, when a concave portion 7 b faces thesensor head 10 a, the amount of eddy current becomes small and theimpedance change in the detection coil 11 becomes small, the sensordetection voltage, therefore, becomes large. It is to be noted that thepower for operating the eddy current sensor 10 is supplied from thesignal processing device 20.

Sensor detection voltage outputted from the eddy current sensor 10 asdescribed above is converted into pulse signals in the signal processingdevice 20. Wheel rotational speed is calculated based on the pulsesignals. That is, sensor detection voltage is firstly sampled atpredetermined sampling frequency by an A/D converter 21, and convertedinto digital value. Hereinafter, this sensor detection voltage after A/Dconversion is referred to as detection data (A/D value).

This detection data is inputted into a signal level determination unit22 and a pulse conversion unit 23. Firstly, the signal leveldetermination unit 22 is going to be described. The signal leveldetermination unit 22 calculates threshold voltages V_(TH) and V_(TL)(i.e. thresholds having hysteresis) based on detection data, and outputsthe voltages to the pulse conversion unit 23. More specifically, thesignal level determination unit 22 conducts a threshold voltage shiftingprocess wherein preset default threshold voltages V_(TH0) and V_(TL0)are shifted (moved) corresponding to detection data.

FIG. 3 is a flowchart showing the threshold voltage shifting processconducted in the signal level determination unit 22. This thresholdvoltage shifting process is continuously conducted while a linearvehicle is traveling on the wheel 3.

When this process is started, firstly in step (to be abbreviated as “S”hereinafter) 110, the maximum value of sensor detection voltage(actually, detection data) V_(max) is obtained. In the subsequent S120,the minimum value of sensor detection voltage (detection data) V_(min)is obtained. In other words, the maximum amplitude value of sensordetection voltage, and the minimum amplitude value of sensor detectionare respectively obtained in S110 and S120.

Subsequently, in S130, actual average voltage V_(av), which is theaverage of the maximum value V_(max) and the minimum value V_(min), iscalculated. Meanwhile, in the signal level determination unit 22,default average voltage V_(av0) (the default average of the presentinvention), which is the average of sensor detection voltage when thedistance between the sensor head 10 a and a convex portion 7 a is thedefault facing distance, is stored. In subsequent S140, shifting amountZ which is the difference between the actual average voltage V_(av)obtained in S130 and the above-mentioned default average voltage V_(av0)is calculated.

Then, in S150, by shifting the default threshold voltages V_(TH0) andV_(TL0) respectively for the shifting amount Z, actual thresholdvoltages V_(TH) and V_(TL) which are the actual threshold voltages, areobtained. That is, the deviation of sensor detection voltage wherein thedistance between the sensor head 10 a and a convex portion 7 a is thedefault facing distance from actual sensor detection voltage isconverted into the difference of average voltage (default averagevoltage and actual average voltage). Subsequently, this difference isdealt as the shifting amount Z of default threshold voltages V_(TH0) andV_(TL0). Actual threshold voltages V_(TH) and V_(TL) obtained as aboveare outputted to the pulse conversion unit 23.

In the pulse conversion unit 23, detection data from the A/D converter21 is converted into pulse signals corresponding to the actual thresholdvoltages V_(TH) and V_(TL), and outputted to a speed calculation unit24. In the speed calculation unit 24, calculation of wheel rotationalspeed is conducted based on the number of pulse signals per unit time orpulse intervals (cycle). The wheel rotational speed obtained as above istransmitted to a brake control unit, not shown, and used for controllingthe disk brake device.

FIGS. 4A and 4B show examples of sensor detection voltage (detectiondata) and pulse output in the wheel speed detection system of thepresent embodiment. FIG. 4A shows a case wherein the sensor head 10 aand a convex portion 7 a are adjacent and the facing distance dtherebetween becomes smaller than the default facing distance d₀. Whenthe facing distance d becomes smaller than the default facing distanced₀ as above, sensor detection voltage becomes smaller than the sensordetection voltage when the facing distance is equivalent to the defaultfacing distance. In the state shown in the drawing, the defaultthreshold voltage V_(TH0) is larger than the maximum value of sensordetection voltage V_(max).

Consequently, in the present embodiment, by execution of the thresholdvoltage shifting process described in FIG. 3, actual average voltageV_(av) is calculated from the maximum value V_(max) and the minimumvoltage V_(min) of actual detection voltage, and the difference betweenthe actual average voltage V_(av) and the default average voltageV_(av0) becomes the shifting amount Z. Hence, the default thresholdvoltage V_(TH0) is shifted for the shifting amount Z to be actualthreshold voltage V_(TH), and the default threshold voltage V_(TL0) isalso shifted for the shifting amount Z to be actual threshold voltageV_(TL). That is, the default threshold voltages V_(TH0) and V_(TL0) arerespectively decreased by |Z|. Then, by eliminating actual sensordetection voltage at actual threshold voltages V_(TH) and V_(TL) to makea pulse, the pulse output shown in the drawing can be obtained.

On the other hand, FIG. 4B shows a case wherein the sensor head 10 a anda convex portion 7 a are distant and the facing distance d therebetweenbecomes larger than the default facing distance d₀. When the facingdistance d becomes larger than the default facing distance d₀ as above,the sensor detection voltage becomes larger than the sensor detectionvoltage when the facing distance is equivalent to the default facingdistance. In the state shown in the drawing, the default thresholdvoltage V_(TL0) is smaller than the minimum value of the sensordetection voltage V_(min).

Therefore, by execution of the threshold voltage shifting processdescribed in FIG. 3, in the same manner described in FIG. 4A, thedifference between the actual average voltage V_(av) and the defaultaverage voltage V_(av0) becomes the shifting amount Z. The defaultthreshold voltage V_(TH0) is shifted for the shifting amount Z to beactual threshold voltage V_(TH), and the default threshold voltageV_(TL0) is also shifted for the shifting amount Z to be actual thresholdvoltage V_(TL). That is, the default threshold voltages V_(TH0) andV_(TL0) are respectively increased by |Z|. Then, by eliminating actualsensor detection voltage at actual threshold voltages V_(TH) and V_(TL)to make a pulse, the pulse output shown in the drawing can be obtained.

As described above, in the wheel speed detection system of the presentembodiment, by shifting preset default threshold voltages V_(TH0) andV_(TL0) for the shifting amount Z, which is the difference between theaverage of sensor detection voltage actually detected by the eddycurrent sensor 10 and default average voltage, actual threshold voltagesV_(TH) and V_(TL) are obtained. Then, according to these actualthreshold voltages V_(TH) and V_(TL), actual sensor detection voltage(detection data) is converted into pulse signals.

Therefore, according to the wheel speed detection system of the presentembodiment, even when the facing distance between a convex portion 7 aand the sensor head 10 a is changed due to a change in axle load orother reasons, a conversion into pulse signals is conducted according toappropriate actual threshold levels V_(TH) and V_(TL) corresponding tothe change of the facing distance. Accordingly, irrelevant to a changein the facing distance, wheel rotational speed can be detectedaccurately.

Moreover, in the present embodiment, a shifting is conducted not only ina way so that default threshold voltages V_(TH0) and V_(TL0) fall withinthe amplitude of actual sensor detection voltage, but also in a way sothat more appropriate shifting amount Z can be obtained based on thedifference between the default average voltage V_(av0) and the actualaverage voltage V_(av). Hence, default threshold voltages V_(TH) andV_(TL) are more appropriately shifted corresponding to actual sensordetection voltage, and a reliable wheel speed detection system can beprovided.

It is to be noted that in the present embodiment, the eddy currentsensor 10 is relevant to the detector of the present invention, thepulse conversion unit 23 is relevant to the pulse converter of thepresent invention, the speed calculation unit 24 is relevant to thespeed calculator of the present invention, and the signal leveldetermination unit 22 is relevant to the threshold shifter of thepresent invention. Additionally, the threshold voltage shifting processin FIG. 3 is relevant to the process that the threshold shifter of thepresent invention conducts.

Second Embodiment

In the above-described first embodiment, the case wherein defaultthreshold voltages V_(TH0) and V_(TL0) are shifted corresponding to achange in the facing distance between the sensor head 10 a and a convexportion 7 a, is described. In the present embodiment, threshold voltagesare not shifted, but sensor detection voltage detected by the eddycurrent sensor 10 (detection data) itself is shifted, and appropriatepulse signals corresponding to a change in the facing distance areobtained.

FIG. 5 is a block diagram showing schematic structure of a signalprocessing device 30 of the wheel speed detection system of the presentembodiment. The signal processing device 30 is used to substitute thesignal processing device 20 of the wheel speed detection system in thefirst embodiment. Sensor detection voltage from the eddy current sensor10 is firstly sampled by the A/D converter 21 at predetermined samplingfrequency, and converted into detection data (A/D value) which isdigital value.

This detection data is inputted into a detection data shifting unit 31,and shifted for predetermined amount. That is, in the detection datashifting unit 31, a detection data shifting process is conducted inorder to shift actual detection data corresponding to the differencebetween the average of the detection data and preset default averagevoltage V_(av0).

FIG. 6 is a flow chart showing a detection data shifting processexecuted in the detection data shifting unit 31. This detection datashifting process is also continuously conducted while a linear vehicleis traveling on the wheel 3. It is to be noted that in this process,S210 to S240 are respectively the same as S110 to S140 of the thresholdvoltage shifting process described in FIG. 3. Thus, the processes ofS210 to S240 are not described here in detail.

That is, the maximum value V_(max) and the minimum value V_(min) of thedetection data are respectively obtained in S210 and S220, actualaverage voltage V_(av), which is the average of the two values, iscalculated in S230, and the difference between the actual averagevoltage V_(av) and the default average voltage V_(av0) is calculated tobe the shifting amount Z in the subsequent S240. Then, in S250, actualdetection data is shifted for the obtained shifting amount Z.

Detection data shifted as above is inputted into the pulse conversionunit 32, and converted into pulse signals. In this pulse conversion unit32, threshold voltages relevant to default threshold voltages V_(TH0)and V_(TL0) of the first embodiment are preset, and above-mentionedshifted detection data is converted into pulse signals according tothese threshold voltages. Then, these pulse signals are inputted intothe speed calculation unit 24, and in the same manner as in the firstembodiment, wheel rotational speed is calculated based on the number ofpulse per unit time or pulse intervals (cycle).

In FIGS. 7A and 7B, examples of sensor detection voltage (detectiondata) and pulse output in the wheel speed detection system of thepresent embodiment are shown. FIG. 7A shows a case wherein the sensorhead 10 a and a convex portion 7 a are adjacent to each other and thefacing distance d therebetween has become smaller than the defaultfacing distance d₀. When the facing distance d becomes smaller than thedefault facing distance d₀ as above, actual sensor detection voltagebecomes smaller than sensor detection voltage when the facing distance dis equivalent to the default facing distance. In the state shown in thedrawing, the default threshold voltage V_(TH0) has become larger thanthe maximum value of the sensor detection voltage V_(max).

Thereby, in the present embodiment, by execution of the detection datashifting process described in FIG. 6, actual average voltage V_(av) iscalculated from the maximum and minimum values of actual detectionvoltage V_(max) and V_(min), and the difference between the actualaverage voltage V_(av) and the default average voltage V_(av0) becomesthe shifting amount Z. Actual sensor detection voltage (detection data)is shifted for the shifting amount Z. That is, default thresholdvoltages V_(TH0) and V_(TL0) are not respectively decreased by |Z| as inthe first embodiment, but detection data itself is increased by |Z|.Subsequently, by making a pulse from the shifted sensor detectionvoltage, the pulse output as shown in the drawing can be obtained.

On the other hand, FIG. 7B shows a case wherein the sensor head 10 a anda convex portion 7 a are distant and the facing distance d therebetweenhas become larger than the default facing distance d₀. When the facingdistance d becomes larger than the default facing distance d₀ as above,sensor detection voltage becomes larger than sensor detection voltagewhen the facing distance d is equivalent to the default facing distance.In the state shown in the drawing, the default threshold voltage V_(TL0)has become smaller than the minimum value of the sensor detectionvoltage V_(min).

Thereby, by execution of the detection data shifting process describedin FIG. 6, in the same manner as described in FIG. 7A, the differencebetween actual average voltage Vav and the default average voltage Vav₀becomes the shifting amount Z, and actual detection data is shifted forthe shifting amount Z. That is, default threshold voltages V_(TH0) andV_(TL0) are not respectively increased by |Z| such as in the firstembodiment, but detection data itself is decreased by |Z|. Subsequently,by making a pulse from the shifted sensor detection voltage, pulseoutput shown in the drawing can be obtained.

Therefore, according to the present embodiment, even if a deviation ofthe sensor detection voltage when the facing distance is equivalent tothe default facing distance from actual sensor detection voltage iscaused because of the change of facing distance between the surface of aconvex portion 7 a and the sensor head 10 a due to a change in axle loador other reasons, the sensor detection voltage (detection data) isshifted toward a direction to restitute the difference, and the sameeffect as the first embodiment can be achieved.

It is to be noted that in the present embodiment, the detection datashifting unit 31 is relevant to the detection signal shifter of thepresent invention, and the detection data shifting process in FIG. 6 isrelevant to the process executed by the detection signal shifter of thepresent invention.

It goes without saying that embodiments of the present invention are notlimited to the above described embodiments, and that variations andmodifications can be adopted within the technical scope of the presentinvention.

For example, in the respective embodiments described above, thedifference between actual average voltage V_(av) and default averagevoltage V_(av0) is set to be the shifting amount Z. Instead, thedifference between the maximum (or the minimum) value of sensordetection voltage, when the facing distance is equivalent to the defaultfacing distance, and the maximum value of the sensor detection voltageV_(max) (or the minimum value V_(min)) can be the shifting amount Z. Asfar as an appropriate shifting amount Z corresponding to a change in thefacing distance can be obtained, the shifting amount Z can be obtainedin various ways.

Additionally, in the first embodiment described above, default thresholdvoltages V_(TH0) and V_(TL0) are preset and these default thresholdvoltages V_(TH0) and V_(TL0) are shifted for the shifting amount Z.Instead of presetting the default threshold voltages V_(TH0) andV_(TL0), threshold voltages can be obtained by calculating thresholdvoltages one by one based on the amplitude (the maximum and maximumvalues) of detection data actually obtained. Specifically, for example,a value which is predetermined level lower than the maximum value ofactual detection data, and a value which is predetermined level higherthan the minimum value of actual detection data can be threshold levels.

Furthermore, in the respective embodiments described above, even whenthe difference between actual average voltage V_(av) and default averagevoltage V_(av0) is small, a shifting is conducted as long as there is adifference. However, when the difference is small and ignorable,conversion into pulse signals can be normally conducted without ashifting. Hence, it can be arranged so that a shifting is not conductedevery time irrelevant to the size of the difference, but a shifting isconducted only when the difference is large and normal pulse conversioncannot be expected. That is, logically, at least when threshold levelsare out of the amplitude range of actual sensor detection voltage, ashifting can be conducted so that threshold levels fit into theamplitude range.

Still furthermore, in the respective embodiments described above,detection data is converted in to pulse signals by the thresholdvoltages having hysteresis. Alternatively, one threshold level withouthysteresis can be used. However, in consideration of noise resistance,it is preferable to use threshold voltages having hysteresis as therespective embodiments described above.

Additionally, other than disposing the concave and convex portion 7 onthe periphery of the rotational surface of the wheel 2 as in theabove-described embodiments, the concave and convex portion 7 can bedisposed, for example, on the lateral face of the wheel 2 (i.e. such asgear figuration), such as in the art disclosed in the document 1described earlier. As long as the working-effect of the presentinvention can be attained, the forming position is not speciallylimited.

The shape of a convex portion 7 a is not limited to the shape in theabove-described embodiments, various shapes can be adopted as long as achange of concavity and convexity can be detected as a difference in theamount of eddy current (consequently, can be detected as change in theimpedance of the detection coil 11).

Furthermore, in respective signal processing devices 20 and 30 of therespective embodiments described above, sensor detection voltage fromthe eddy current sensor 10 directly goes through A/D conversion.Alternatively, a high frequency wave elimination filter, for example,can be disposed before the A/D converter 21 to cut components of highfrequency waves. In this way, the reliability of this system can beimproved.

INDUSTRIAL AVAILABILITY

As described above, according to the wheel speed detection system of thepresent invention, even when the distance between the eddy currentsensor and a convex portion changes due to various factors such as achange in load on a wheel or an error in assembly, it is possible todetect wheel speed accurately.

1. A wheel speed detection system comprising: a rotator configured torotate on an axle center of a wheel together with the wheel, and pluralconcave and convex portions formed on a periphery of the rotator along arotational direction with predetermined intervals therebetween; a sensorhead disposed so as to face a surface of the convex portion with certaindistance therebetween, and constituted with a coil to generate alternatecurrent magnetic field therearound under supply of alternate current; adetector configured to excite the coil by supplying alternate current togenerate eddy current on the concave and convex portions, and outputsalternate current detection signals corresponding to a change in anamount of the eddy current generated with rotation of the rotator; apulse converter configured to convert the alternate current detectionsignals into pulse signals according to preset threshold levels; and aspeed calculator configured to calculate rotational speed of the wheelbased on the pulse signals, the wheel speed detection system furthercomprising a threshold shifter configured to shift the threshold levelscorresponding to actual facing distance between the surface of theconvex portion and the sensor head, wherein the pulse converter isconfigured to conduct conversion into the pulse signals according to theshifted threshold levels when the threshold levels are shifted by thethreshold shifter, and wherein the threshold shifter is configured toobtain a difference between a default average, which is an average ofthe alternate current detection signals when a preset facing distancebetween the surface of the convex portion and the sensor head is thecertain distance, and an average of the alternate current detectionsignals actually outputted from the detector, and shifts the thresholdlevels corresponding to the difference.
 2. The wheel speed detectionsystem as set forth in claim 1, wherein the threshold shifter isconfigured to shift the threshold levels so that the threshold levelsfall within amplitude range of alternate current detection signalsoutputted from the detector.
 3. The wheel speed detection system as setforth in claim 1, wherein the threshold levels are constituted with twothreshold levels having hysteresis, and wherein the threshold shifter isconfigured to shift the two threshold levels while retaining an amountof the hysteresis.
 4. The wheel detection system as set forth in claim1, wherein the wheel speed detection system is (1) mounted on a vehicleof a railway with primary side on ground system, the vehicle beingconfigured for propulsion by magnetic interaction generated betweenpropulsion coils disposed along a track on a ground and a magnetic fieldsystem mounted on the vehicle by controlling power supply to thepropulsion coils, and (2) configured to obtain rotational speed of thewheel disposed on the vehicle.